Modern silviculture often requires the planting of large numbers of genetically identical plants that have been selected to have advantageous properties. Production of new plants by sexual reproduction, which yields botanic seeds, is usually not feasible. Asexual propagation, via the culturing of somatic or zygotic embryos, has been shown for some species to yield large numbers of genetically identical embryos, each having the capacity to develop into a normal plant.
Somatic cloning is the process of creating genetically identical plants from plant tissue other than male and female gametes. In one approach to somatic cloning, plant tissue is cultured in an initiation medium that includes hormones, such as auxins and/or cytokinins, to initiate formation of embryogenic tissue, such as an embryogenic suspensor mass, that is capable of developing into somatic embryos. Embryogenic suspensor mass, or ESM, has the appearance of a whitish translucent mucilaginous mass and contains early stage embryos. The embryogenic suspensor mass is further cultured in a multiplication medium that promotes multiplication and mass production of the embryogenic suspensor mass. The embryogenic suspensor mass is then cultured in a development medium that promotes development and maturation of cotyledonary somatic embryos that can, for example, be placed on germination medium to produce germinants, and subsequently transferred to soil for further growth, or alternatively, placed within manufactured seeds and sown in soil where they germinate to yield seedlings. Manufactured seeds are described, for example, in U.S. Pat. Nos. 5,564,224; 5,687,504; 5,701,699; and 6,119,395.
A continuing problem with somatic cloning of conifer embryos is stimulating efficient and cost-effective formation of somatic embryos that are capable of germinating to yield plants. Preferably, conifer somatic embryos, formed in vitro, are physically and physiologically similar, or identical, to conifer zygotic embryos formed in vivo in conifer seeds. There is, therefore, a continuing need for methods for producing viable conifer somatic embryos from conifer embryogenic cells.